C4 plants, such as Zea mays, contain two photosynthetically distinct cell types (mesophyll and bundle sheath) which differ in their structure and function. Our long range goal is to understand the regulatory mechanisms controlling the differential expression of nuclear-encoded chloroplast proteins involved in C4 photosynthesis. With this in mind we have isolated the two cell types from maize and analyzed their polypeptide components by immunochemical methods. We have shown by this analysis that the enzyme ribulose-1,5-bisphosphate (RuBP) carboxylase is localized in the chloroplast stroma of bundle sheath cells and absent from mesophyll cells. Furthermore, we have shown that the differential localization of the carboxylase small subunit is due to the absence of translatable mRNA encoding this protein in mesophyll cells. In this study we propose to investigate the differential expression of the RuBP carboxylase small subunit gene in mesophyll and bundle sheath cells using recombinant DNA technology to isolate a cDNA clone complementary to the small subunit mRNA. This cloned cDNA will be used as a molecular hybridization probe to localize complementary RNA sequences in the two cell types. Such studies will allow us to determine at what level of gene expression (transciption, RNA processing, nucleo-cytoplasmic transport, or differential mRNA turnover) the small subunit gene is controlled in mesophyll cells. In addition, we will be able to isolate and characterize genomic sequences coding for the carboxylase small subunit from these two differentiated plant cell types.